The use of optical fibers in telecommunication networks is an established technology. Tunable optical filters having a high tuning speed, a wide tuning range and of low cost are required to utilize this technology effectively.
Polarization-sensitivity is a general characteristic of many optical filters and represents a serious limitation for use in telecommunication applications, since transmission of light down an optical fiber generally results in severe polarization mixing due to the nature of the physical changes in the conditions down the fiber, such as bending, temperature, stress and other factors. Therefore, the lack of polarization independence in filtering applications in optical fibers, for example in wavelength division multiplexing WDM, where input polarization is unknown and generally uncontrollable, is a serious problem.
For arbitrary polarized input, an optical filter typically decomposes the polarization into two components, one of which interacts with the ordinary index of refraction no, and the other of which interacts with the extra-ordinary index of refraction ne. In such cases, the filter transmits two peaks, one associated with each polarization component. If a polarizer is used to eliminate the unwanted resonant peak, such as that arising from the ordinary wave, for arbitrary input polarization from a fiber, up to half of the input power may be lost.
A number of prior art patents have addressed the issue of polarization independence in optical components, and especially in filters, some of them relying on the use of polarization diversity at the component input. Among such prior art patents are U.S. Pat. No. 6,522,467 to Li, et. al., for “Polarization insensitive liquid crystal tunable filter”; U.S. Pat. No. 5,781,268 to Liu, et. al., for “Polarization-insensitive Fabry-Perot tunable filter”; U.S. Pat. No. 6,081,367 to Yokoyama, et. al., for “Optical filter module and optical amplifier using the same”; U.S. Pat. No. 5,710,655 to Rumbaugh, et. al., for “Cavity thickness compensated etalon filter”; U.S. Pat. No. 5,321,539 to Hirabayashi, et. al., for “Liquid crystal Fabry-Perot etalon with glass spacer”; and U.S. Pat. No. 5,111,321 to Patel, for “Dual-polarization liquid crystal etalon filter”.
Some of the solutions presented are considered to be bulky or complex to construct, or have non-negligible insertion losses, such that there still exists a need for polarization insensitive, compact, broadband optical filter components having high optical throughput.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.